Edible coatings are used in foods to minimize the migration of components within the food system or between the food and its surrounding environment. For example such coatings prevent the diffusion of water, fats and/or oxygen into, out of, or within the food system. Of these, reducing moisture migration is the most important requirement in most cases. A change in either direction of moisture levels or water activity does not have to be very large to be very detrimental for the food quality. Moisture loss or uptake in a food can have dramatic effects on the texture, stability or yield of the food product. Moisture uptake can reduce or eliminate crispness, can speed enzymatic or chemical deterioration of flavors or nutrients, and can impair the food's structural integrity.
Edible barriers can also reduce gas permeability in foods. Oxygen uptake by a food often results in deleterious reactions which affect its flavor, nutritional quality and acceptability. In complex food systems, the migration of water or lipids within the food itself may have a detrimental effect on the perceived quality. Also, in processing operations, such as deep fat frying, coatings may reduce the migration of a processing aid, i.e., fat, into the food. In addition, barriers may be used to minimize the migration or loss of other additives, such as colors, flavors, preservatives, antioxidants, etc. . . Edible coatings also can be used to impart structural integrity to the surface of a food, making it less susceptible to mechanical damage.
Kester and Fennema ("Edible Films and Coatings: a Review," Food Technology, (1986), 40(12):47-59) reviewed the use of edible films and coatings, and the current state-of-the-art coating compositions. The most common edible barrier materials, in the category of those which would not be readily apparent to the consumer, include polysaccharides, e.g., alginate, pectin, carrageenan, starch, starch hydrolysates and cellulose derivatives; lipids, e.g., acetylated monoglycerides, natural waxes and surfactants; and proteins, e.g., gelatin, casein, serum albumin, ovalbumin, wheat gluten and zein, plus combinations of these. Another common edible barrier not encompassed in the above catagories is shellac. Films containing wheat gluten, which contains the wheat prolamine gliadin, and films containing zein have only received limited attention as edible coatings. Of the protein-based films, zein films have been found to possess relatively good water-barrier properties. Guilbert, "Technology and Application of Edible Protective Films," In: Food Packaging and Preservation. Theory and Practice, M. Mathiouthi (Ed)., Elsevier Applied Science Publishing Co., London, England, (1986), pp. 371-391.
Prolamines are characterized by their solubility in aqueous alcohol mixtures, or in aqueous mixtures of extreme pH (less than pH 2 or greater than pH 10) and therefore, most of the applications of prolamine films are from solutions of prolamines in alcohol and other solvent mixtures or water-based solutions having extreme pH's. The disadvantage of these coating systems in foods, hence the reluctance by the food industry for use of prolamine film systems, is that the solvents and/or pH levels are often incompatible with and difficult to remove from the food. Also, the use of organic solvents poses safety issues with the emission of vapors during the curing of the films, with the fire hazard that they pose, and with the possible residuals they may leave in the food.
Non-edible films based on prolamines have been described. Hansen (U.S. Pat. No. 2,047,961) describes a non-edible prolamine-based film which is formed by reacting the protein in an aqueous-alcoholic solution with formaldehyde and mixing the resultant with an alcohol soluble phenol-formaldehyde-type resin and a plasticizer. In U.S. Pat. Nos. 2,115,716 and 2,115,716, Hansen describes modifications to the solvent-based film systems in which he incorporates a plasticizing system comprised of amino acid esters to enhance the moisture resistance of the films and a high boiling (greater than 120.degree. C.) organic solvent which balances the evaporation rates of the more volatile components in the coating mixture. Veatch (U.S. Pat. No. 2,134,769) describes improving the water resistance of non-edible zein films by the addition of waxes to alcoholic zein coating compositions, but required the addition of benzene and/or toluene to dissolve the waxes. Veatch (U.S. Pat. No. 2,194,337) also describes a non-edible zein film with improved grease resistance by incorporating urea and glycol in the alcohol mixture prior to casting the film. In U.S. Pat. No. 2,229,028 Sturken uses zein in an alcohol based coating composition with formaldehyde, and cures the films on paper sheets by the simultaneous application of heat and pressure. In U.S. Pat. No. 2,250,041 Sturken further describes a non-aqueous zein coating composition with improved plasticizers. In U.S. Pat. No. 2,311,485 Sturken introduces the use of sorbitol and triethanolamine as plasticizers for non-edible zein films deposited by evaporation from an ethanol solution. Evans and Manley (U.S. Pat. No. 2,437,8946) report that lactamides are excellent plasticizers for prolamine films, and that these may be hardened by reaction with an aldehyde. In U.S. Pat. No. 2,285,758 Sturken discloses a process for treating a zein film to give it water resistance. In this process, a zein film is brought into direct contact with steam.
Solvent-based coating compositions containing zein in solution with relatively large amounts of water have a tendency to gel on standing. Coleman (U.S. Pat. No. 2,185,123) describes a stable solution of zein in 95% alcohol to which an auxiliary solvent which is a lacquer solvent or a lacquer plasticizer, has been added. In U.S. Pat. No. 2,185,124 Coleman further describes a substantially non-aqueous coating composition which resists gelation, wherein zein is dissolved in alcohol containing not more than 5% water. Evans and Manley ("Stabilizing Zein Dispersions Against Gelation," Industrial and Engineering Chemistry, 1943, 35(2):230-232) and Evans (U.S. Pat. No. 2,402,128) stabilized the solvent solutions against gelation by using a heat treatment and reaction with aldehydes.
In U.S. Pat. No. 2,143,023, Meigs describes a derivatization of zein in which the zein is reacted together with formaldehyde and a secondary dialkylamine having less than nine carbon atoms. The zein derivative is soluble in dilute aqueous acids and may be used in non-edible coating compositions. In U.S. Pat. No. 2,377,237 James uses surfactants to disperse the zein in a predominantly aqueous phase.
Solvent-based prolamine films and coatings have been described for edible applications. In U.S. Pat. No. 2,791,509 Cosler discloses the use of zein as a surface coating for confectioneries. In these films, zein and an acetylated glyceride plasticizer are dissolved in a mutual organic solvent, preferably ethanol. The solution is deposited on the confectionery surface and the film is set by evaporation of the solvent. In U.S. Pat. No. 3,653.925 Anker, Foster and Loader disclose a coating formulation for foods comprised of wheat gluten, and optionally zein or soy protein isolate, dispersed in an alkaline, aqueous alcohol and plasticized with glycerol.
In food applications, several protein films such as gelatin, casein and zein, have been shown to have good water and/or lipid barrier properties. For example, zein provides a glossy surface possessing both good lipid and moisture barrier protection. The use of zein has been limited to the nut and confection industry, however, due to the need for organic solvents and/or extreme pH's.